The present invention relates to filters, and particularly to underdrains in gravity filters. More particularly, the present invention relates to underdrains in gravity filters having a cap or plate supporting a filter media.
Gravity filters frequently employ underdrain blocks to channel fluid away from the filter bed and to direct backwash fluids evenly into the filter bed during a backwash cycle. Underdrain blocks are assembled end to end forming rows, also known as laterals. Several laterals are placed next to one another to define the filter bottom. A filter media (e.g., granular activated carbon, anthracite, coal, magnesium oxide, ilmenite, sand including garnet, silica or quartz, etc.) placed on top of the laterals completes the filter bed and performs a majority of the filtration.
In operation, the filter bed receives a flow of fluid to be filtered and passes it through the filter media to the underdrain blocks. The size and arrangement of the filter media allows it to filter out the undesired contaminates, with finer filter media being able to filter out smaller contaminates. The flow, or filtrate, enters the underdrain blocks through openings generally provided in the tops of the underdrain blocks. The underdrain block laterals provide a convenient channel or flow path, allowing removal of the filtrate from the filter bed.
Periodically, a backwash cycle is initiated to clean the filter media. During a backwash cycle, fluid flow reverses, thus flowing into the underdrain blocks and upwardly through the filter media to loosen and remove contaminates.
It is desirable to have large openings in the filter blocks to allow free filtrate flow into the blocks. However, large openings are incapable of supporting the filter media and instead allow the media to pass into the underdrain blocks, which is undesirable. Small openings on the other hand, are capable of supporting the filter media, but are easily plugged by the filter media disposed above and upstream of the underdrain blocks. Also, small openings provide greater restriction to flow than large openings even when unclogged. Therefore, it is desirable to provide an underdrain block capable of supporting the filter media above, while simultaneously reducing the likelihood of media entering the underdrain block, and minimizing plugging of the openings. It is also desirable to provide an underdrain block having a high percentage of open surface area, thereby allowing a free fluid flow therethrough.